Winding machines have a long history. Such machines are used in various applications to wind linear media, such as wire or cable, onto a spool to form a coil. In some embodiments, conductive coils can be formed by winding an electrical conductor around a coil form such as a spool or a core. An example of a conductive coil can be seen in a rotor of an electric motor, where the rotor includes an insulated resistive conductor (such as a copper wire surrounded by electrical insulation) wound around an iron core. Winding machines are also commonly used to wind coils of wire or other material onto large spools for storage or transport.
While prior art winding machines work reasonably well for materials such as metal wire, there are a number of delicate linear media types today that are too fragile for prior art winding machines and techniques. Examples of such delicate media include low and high temperature superconducting wire, very fine conventional wire, fiber optic wire, thin strands of carbon based fiber, smart fabrics, or extremely dense fine fiber matrices for impact or extreme environment protection. While the method and apparatus of the present invention could be applicable to any of these delicate media types, much of the present discussion will focus on superconductor wire, particularly brittle superconductor types such as reacted magnesium diboride (MgB2) or niobium-3 tin (Nb3Sn) wire.
A superconductor is a material that exhibits extremely low electrical resistance at low temperatures. Superconducting cables and wires are used in a variety of applications, including the production of powerful electromagnets used in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR) spectroscopy, mass spectrometers, and beam steering magnets for particle accelerators. Superconducting magnetic coils, like most magnetic coils, are formed by wrapping an insulated conducting material around a form defining the shape of the coil. When the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current-carrying performance of the conductor is greatly increased and large magnetic fields can be generated by the coil.
To wind a cable or wire into a coil, the cable or wire must be bent. The smaller the coil, the more the cable, or wire must be bent. When a superconductor or superconducting cable is bent, strain is induced on the superconducting filaments. Since many superconductors are brittle, bending them can cause them to break. When superconductor wires are wound onto a spool, for example by a prior art winding machine, the stress on the superconducting filaments can be great enough that the superconducting properties of the wires can be destroyed. As a result, for a given superconductor or superconducting cable, there is a lower limit on the radius of curvature to which the superconductor or superconducting cable can be wound within the magnet system, dependent on the irreversible strain of the superconducting filaments within the superconductor or superconducting cable.
Because of the difficulties in handling certain particularly brittle low-temperature superconducting cables/wires, a “wind-then-react” method is often used, whereby the unreacted precursor to a superconductor is wound in a coil around a form or spool and then the entire spool is processed with high temperatures and an oxidizing environment. This results in the conversion of the precursor material into the desired superconductor material already formed into the desired coil shape.
However, this approach has several disadvantages in many cases. Because the precursor material must be heated after the wire is coiled onto a magnet system and spool, all of the components of the magnet system must be able to withstand with the high temperatures used during the formation of the superconducting phase. This means, for example, that the magnet system cannot include aluminum or its alloys since these melt at the temperatures used during formation of the superconducting filaments. It is also difficult and expensive to apply insulation to a wound coil in order to prevent electrical current flow between the turns. Finally, the “wind-then-react” method also leads to storage difficulties and added expense because the superconducting wire cannot be easily prepared and stored ahead of time (since it must be formed onto the spool or system in which it will be used).
One particular problem area is seen in the production of MRI machines, large motors or generators, or large accelerator magnets, which all require a coil of superconducting wire that is several kilometers in length and weighs hundreds to thousands of pounds. The sheer size of the required coil presents a number of difficulties when using a “wind-then-react” method since the entire coil must be placed in an oven for processing.
In contrast, a “react-then-wind” method of production would provide a number of advantages, including decreased manufacture and storage costs and allowing for a broader range of materials to be used with the magnet system. But despite these known advantages, the difficulties in handling reacted superconductor wires without damage—especially for lower cost superconducting materials like magnesium diboride—has prevented the “react-then-wind” method from gaining widespread commercial acceptance.
The problem with all prior art winding systems of which Applicant is aware is that the means of passive or even active tension control is still too coarse for the most delicate linear media requirements and hence often damages the media. A number of prior art systems use a dancer pulley for tension control. However, the mechanical action of tension control using such a dancer pulley under high acceleration or deceleration profiles places an unacceptable impulsive force on the linear media and also often damages the media. Even for the closed loop control solutions implemented in the prior art, the methods used for tension measurement are either too inaccurate (such as using overall system weight) or too damaging to the media (using three-pulley tensiometers with small pulleys and reverse bends). Unfortunately, there is no current winding system that is capable of winding today's most delicate linear media, such as the extremely delicate, low-temperature superconducting reacted magnesium diboride (MgB2) and niobium-3 tin (Nb3Sn) based low temperature superconductor wires or manufacturing quality high temperature superconducting wires such as yttrium barium copper oxide (YBCO), bismuth strontium calcium copper oxide (BSCCO), or larger diameter fiber optical wire, without continual human intervention. This leads at best to long process times and poor quality control, as well as difficulties in meeting manufacturing repeatability standards, and at worst to media that is so damaged by the winding process that it can no longer be used for its original purpose or that has an extremely shortened operational life from poor media handling induced fatigue.
Thus, there is a need for an improved method and apparatus for handling delicate linear media and for winding such delicate media from or into a coil for use or storage.